Definition - What does Extensible User Interface Language (XUL) mean?

Customization of user interfaces tends to be challenging when applied to operating system platforms from various vendors. The Mozilla Foundation developed XUL to resolve this issue.

Techopedia explains Extensible User Interface Language (XUL)

XUL is a programmer platform favorite, largely due to its simplicity. It's easier to use than earlier interface programming tools, and multiple changes can be made to operating system platforms by adding just a few XUL code lines. Prior to XUL, this would have taken a great deal of time and effort.

XUL has also simplified work for new developers with no prior programming experience. This has helped to increase the platform’s popularity.

XUL is an application of XML, CSS, the document object model (DOM) and HTML. As such, XUL is highly flexible. Documents must be partially platform-specific to specify reference defaults and preferences. However, this does not add significant development overhead.